Results: Mean percent decrease in GTV for the population is 40.7% (5.2-80.5%). Changes in lung V20 resulting from assumed regression dynamics are < 3% (0.3-2.9%) for all patients and margins. 7 of 9 patients exhibit < 3% differences (0.4-2.6%) in D95 PTV for all margins. An average change of 52.2% (50.7-54.2%) and 5.4% (4.6-6.3%) in D95 PTV for all margins is observed for the remaining 2 patients respectively. The patient with the greatest change in D95 PTV also exhibited a large amount of tumor-volume regression (80.5%).

Conclusions: For 3D conformal plans, V20 of ipsilateral lung and D95 PTV are relatively insensitive to DIR deviations associated with regression dynamics. However, deviations in D95 PTV can be large in the presence of large regression.